The present invention relates to a method and to a device allowing a sound source to be detected and/or located.
The present invention has many applications, among which mention may be made of the field of the non-destructive testing of solid objects by the methods referred to as acoustic emission, illustrations of which may be found in the work "Essais non destructifs. L'emission acoustique--Mise en oeuvre et applications" [Non-destructive tests. Acoustic emission--implementation and applications] by James Roget, AFNOR-CETIM, 1988.
In these acoustic emission methods, the object being studied is subjected to external stresses, which may be mechanical or thermal, and sensors around the object are used to detect possible cracking or micro-noise which are generated by these stresses and reveal structural defects of the object. One difficulty with these methods is that the level of the signal representing a defect may be very low and hard to distinguish from the background noise which accompanies it.
The invention applies more generally to the detection of sound sources which may be of low absolute or relative level, in the signal collected by acoustic sensors. The acoustic propagation medium should not be understood as being limited to solids. It may also be fluid (liquid or gas) or contain interfaces.
In order to overcome the limitation of passive methods of sound source location in the presence of a low signal-to-noise ratio, it is known to combine them with correlation and/or triangulation techniques in order to search for the sources and identify them.
However, these techniques also have limitations. On the one hand, if the propagation medium causes a great deal of perturbation (refraction, multiple reflections, etc.) for acoustic waves, the various sensors of the array receive acoustic signals which have lost all correlation, which makes it almost impossible to detect and locate the sound source precisely. On the other hand, in a very noisy environment (for example a flowing fluid or the marine environment), the signal to noise ratio may be so small that it remains impossible to detect and locate the sound source even with sophisticated correlation calculations.
A method is furthermore known (EP-A-0,383,650 or U.S. Pat. No. 5,092,336) for detecting and locating a reflecting target, according to which the medium is illuminated from at least one transducer emitting a beam referred to as the illumination beam, the signals reflected by the medium are collected using a plurality of transducers belonging to an array, and they are stored; the echoes coming from a determined zone of the medium are selected by windowing; the signals are time reversed and are re-emitted; and the above operations are repeated. In this method, the target possibly detected is generally the most highly reflecting object in the zone in question (calculus in the case of lithotripsy applications, structural defects in the case of non-destructive testing, etc.). If a plurality of reflecting objects contribute to the measured signal, one or more iterations of the process allow selective focusing on the target.
This method gives an already considerable improvement in the detection and location capacities, particularly in media having spatial variations in the acoustic properties such as the speed of the waves (see "Time Reversal of Ultrasonic Fields--Part I: Basic Principles", by M. Fink, IEEE Trans. on UFFC, Vol. 39, No. 5, September 1992, pages 555-566). Under certain conditions, however, the illumination which the illumination beam produces at the target may not be sufficient to produce reflection which is strong enough to be detectable by the array of sensors. On account of this limitation, provision is generally made for the illumination beam to be a brief pressure pulse, allowing wavefront or pulse echo detection in the collected signal (see EP-A-0,591,061 or U.S. Pat. No. 5,428,999). However, this arrangement may be insufficient, in particular when the signal-to-noise ratio is very low, or when there are large aberrations between the transducers and the target.
Furthermore, it could in certain cases happen that the target of interest is not the most highly reflecting object in the explored zone. In such cases, the known methods with time reversal may fail. For example, in non-destructive testing, the contribution from a structural defect to the echo signals may be fully drowned in those from the surfaces or interfaces of the article being examined, this problem requiring special arrangements such as those described in EP-A-0,541,434 or in U.S. Pat. No. 5,431,053.
When the target of interest is a sound source, it is not necessarily the most highly reflecting object in its environment, which limits the effectiveness of the known methods with time reversal in overcoming the deficiencies of the passive listening methods. For example, if hydrophones are used to search for the location of a gas leak along a sub-marine gas pipeline, the gas pipeline section containing the leak will not a priori be more highly reflecting than the other sections.
The object of the present invention is, in particular, to provide a method for detecting and/or locating a sound source which meets practical requirements better than those previously known, in particular in so far as it allows a source to be detected in a noisy medium and/or one exhibiting reflections and multiple refractions which defeat the prior methods.